A method using an internal resistance or an internal impedance of a secondary battery is known to detect a state of health (SOH) and a state of function (SOF) of the secondary battery. However, the internal resistance of the secondary battery is a composition of a plurality of resistance components, and therefore, it is necessary to find by decomposing each resistance component to detect the state of the secondary battery with high accuracy, and in particular, it is necessary to find a reaction resistance with high accuracy. As technologies to enable the above, for example, technologies in Patent Document 1, and Patent Documents 2, 3 are disclosed.
Here, in the technology disclosed in Patent Document 1, pulse discharge of the secondary battery is performed at a constant voltage for the predetermined number of times by a frequency of 100 Hz or more, a voltage difference being a difference between a voltage before a start of the pulse discharge and a voltage just after an end thereof is calculated, and the state of function or the state of health of the secondary battery is judged from the voltage difference.
Besides, in the technology disclosed in Patent Document 2, a voltage/current data of an on-vehicle secondary battery of a normal real vehicle environment is acquired not by the pulse discharge, then the voltage/current data is Fourier transformed, a frequency decomposition is performed, and impedance spectrum is found. Then, a constant fitting of an is equivalent circuit model of the objective secondary battery is performed by using the found impedance spectrum, and a resistance component and a double layer capacity component in the secondary battery are decomposed to be found.
Further, in the technology disclosed in Patent Document 3, a voltage/current data of an on-vehicle secondary battery is acquired, constants of an equivalent circuit model of the objective secondary battery are online estimated by using a Kalman filter calculation and so on, and the resistance component and the double layer capacity component in the secondary battery are decomposed to be found.